1. Technical Field
The present invention relates to a method for feeding an unbalanced, three-phase current into a three-phase AC voltage system. The present invention also relates to a corresponding apparatus for feeding an unbalanced current into a three-phase AC voltage system and to a wind power installation provided with such an apparatus.
2. Description of the Related Art
Electrical power is largely distributed in a three-phase AC voltage system, in particular from power producers, such as large power stations, wind power installations or the like, to the consumer. In particular, the consumers adapt in this case to an AC voltage system having particular properties. These include a particular frequency and phase of the individual voltages, the voltage amplitude of each phase and finally also a certain balance of the AC voltage system. Each phase of the three-phase system ideally has, for example for the European interconnected grid system, a root-mean-square value of the voltage of 235 V, a frequency of 50.0 Hz and a phase angle with respect to the respective other two phases of 120° and 240°. Compliance with such properties is an important requirement and deviations are only permissible in certain limits. Excessive deviations may both threaten the stability of the respective three-phase AC voltage system and cause damage in the case of sensitive consumers.
In order to ensure that the required properties are complied with, in particular to ensure that the three-phase AC voltage system is balanced, power producers, in particular large power stations, feed this AC voltage system in a balanced manner. Larger consumers such as factories with large machines must ensure that the AC voltage system is not loaded in an unbalanced manner or is loaded in an unbalanced manner only to a very small extent.
For small consumers, it is assumed that these, on the whole, load the AC voltage system substantially only in a balanced manner for statistical reasons alone.
Nevertheless, the situation may arise in which there is unbalanced loading or possibly unbalanced feeding. An unbalanced system may be the result, at least in sections. In this case, it is desirable or even necessary—depending on the severity of the unbalance—to compensate for the unbalance in the AC voltage system. Large power stations may often not carry out such compensation because they often feed the system by means of a synchronous generator which is directly coupled to the AC voltage system. It is virtually impossible to individually intervene in the individual phases of the synchronous generator during operation and if required.
In the case of feeding by a wind power installation by means of a full converter, it would be conceivable, in principle, to predefine, produce and feed in an unbalanced three-phase alternating current if the full converter used has such a capability. In such a case, each bridge branch of the full converter feeds a different amount of current into the AC voltage system.
Such feeding-in of different currents may result in an uneven load and possibly excessively large loads. If the full converter is operated up to the permissible power limit and feeds the system in an unbalanced manner in this case, this may mean that the load limit for an individual bridge branch is exceeded. Increased ageing of the components or even an acute malfunction can accordingly be expected.